Recently, the display performances of liquid crystal display devices have been improved to the point that more and more manufacturers use them in TV receivers, for example. The viewing angle characteristic of liquid crystal display devices has been improved to a certain degree but is not satisfactorily in some respects. Among other things, there is still a high demand for improvement of the viewing angle characteristic of a liquid crystal display device that uses a vertical alignment liquid crystal layer (which is sometimes called a “VA mode liquid crystal display device”).
A VA mode liquid crystal display device which is currently used for a TV set with a big screen, for example, adopts an alignment division structure in which multiple liquid crystal domains are formed in a single pixel region to improve the viewing angle characteristic. An MVA (multi-domain vertical alignment) mode is often adopted as a method of forming such an alignment division structure. The MVA mode is disclosed in Patent Document No. 1, for example.
Specifically, according to the MVA mode, an alignment control structure is provided on each of the two substrates, which face each other with a vertical alignment liquid crystal layer interposed between them, so as to contact with the liquid crystal layer, thereby forming multiple domains with mutually different alignment directions (i.e., tilt directions), the number of which is typically four, in each pixel region. As the alignment control structure, a slit (as an opening) provided in an electrode or a rib (as a projection structure) may be used, thereby creating an alignment controlling force from both sides of the liquid crystal layer.
If such a slit or rib is adopted, however, the alignment controlling force will be applied onto liquid crystal molecules non-uniformly within a pixel region because the slit or rib has a linear structure unlike the situation where the pretilt directions are defined by an alignment film in a conventional TN (twisted nematic) mode LCD. As a result, the response speed may have a distribution unintentionally. In addition, since the transmittance of light will decrease in the areas with the slits or ribs, the brightness of the screen will decrease, too.
To avoid such a problem, the alignment division structure is suitably formed by defining the pretilt directions with an alignment film for a VA mode liquid crystal display device, too. However, the present inventor discovered that if such an alignment division structure was provided, alignment disorder specific to a VA mode liquid crystal display device would occur to debase its display quality. Specifically, an area that looks darker than the other areas (i.e., a dark line) would be produced parallel to, and in the vicinity of, an edge of a pixel electrode, thus causing a decrease in transmittance.
A structure for minimizing such a decrease in transmittance is proposed in Patent Document No. 2. In the structure proposed in Patent Document No. 2, in a liquid crystal display device in which an alignment division structure is formed by an alignment film, ribs are provided for its counter substrate (i.e., a substrate that faces an active-matrix substrate with pixel electrodes). Those ribs are arranged in areas corresponding to portions of the edges of the pixel electrodes around which dark lines are produced (which are called “edge portions” in Patent Document No. 2), i.e., arranged to face the edge portions. By providing such ribs, the dark lines can be made thinner and driven out of the pixel regions, and therefore, the decrease in transmittance can be minimized.